The present invention relates to a method for producing multichannel plates as amplifiers for optical images or other two-dimensional signal patterns by means of secondary electron multiplication and to the use of a stack of multichannel plates produced according to this method.
It is known to amplify optical images or other two-dimensional signal patterns by means of a so-called multichannel image amplifying plate, channel multiplier plate, or microchannel plate. Such a plate is composed of a glass plate approximately 1 mm in thickness which is encased in an evacuated vessel and is penetrated by a plurality of closely adjacent channels, each approximately 30 microns in diameter, extending perpendicularly or obliquely to the major surfaces of the plate. By using lead oxide containing glasses and a subsequent treatment with reducing gases at elevated temperatures, the walls of the channels are made weakly electrically conductive.
The application of a voltage of about 1000 volt between the metal coated surfaces of the plate produces a potential gradient in the channels, thus imparting the characteristics of a secondary electron multiplier to each channel.
If the channels are given an oblique orientation, collision of primary particles with the channel walls, and thus the desirable release of electrons, is enhanced. Additionally, this channel orientation permits assembly of a stack of plates having a zigzag channel structure which suppresses the undesirable acceleration of parasitic ions. A similar effect can be realized by slightly curving the channels.
Several manufacturing methods are known for such multichannel plates: see, for example Michael Lampton, Spektrum der Wissenschaften [Science Spectrum], January 1982, pages 44-55, which is a translation of an article published in Scientific American, November, 1981. The uses of such multichannel plates are also discussed in this publication.
In the so-called metal core process, a fine, uniform wire is covered with heated glass and wound around a polygonal drum. Individual blocks are then cut out of the coil and the glass coatings of the wires are melted together. Thereafter, the block is cut into thin wafers from which the wire cores are removed by etching. A significant drawback of the described metal core process is found in the fact that the metal cores, and thus the channels, although they have uniform diameters, vary considerably in their spacing from one another.
In another manufacturing process, fine parallel grooves are etched photolithographically into the surfaces of thin glass plates. The plates are stacked in such a manner that the grooves of superposed plates together form the desired channels. Then, the plates are melted together to form blocks from which the multichannel plates are then cut. The advantage of this method is that the distances between the grooves can be regulated with precision during the photolithographic etching. Also, in this method, the channels can be made to be relatively slightly curved or given a zigzag shape. However, it has been found that it is hardly possible to monitor width and depth of the grooves during etching and during the melting process. The result is that the multichannel plates distort the images too much during amplification so that the process finally was found to be unusable.
Today, multichannel plates are usually produced according to the so-called double drawing process. Hollow glass cylinders or glass cylinders filled with a more easily soluble glass are drawn into glass filaments which are bundled, melted and drawn further, whereupon the procedures of bundling and melting are repeated. The final bundle is cut into plates of approximately 1 mm thickness, with the cores of the more easily soluble glass, drawn down to a diameter of about 30 microns, being dissolved out. Due to the manufacturing principle involved, certain fluctuations in cross section and position of the channels are unavoidable in this double drawing process as well.
Variations in cross section and position of the channels in such multichannel plates prevent or make more difficult the precise association of other optical and/or electrical components produced by microproduction methods with the individual channels or channel groups of the image amplifier. However, such an association is important, for example, for the separate further electrical processing of electrical currents furnished by the individual channels or groups of channels. The fluctuations in cross section and position of the channels in the prior art multichannel plates are also responsible for the fact that considerable losses in resolution arise when the above-mentioned plate stacks are assembled to have the zigzag channel structure.
DE-OS [Federal Republic of Germany Laid-Open Application] No. 3,150,257 and DE-PS [Federal Republic of Germany Patent] No. 2,414,658 disclose layered multichannel plates for image amplifiers employing dynodes in the form of perforated dynode plates wherein the preferred method for producing the channel system is the photoetching technique. The dynode material, e.g. a BeCu-Le alloy is etched in through illuminated and developed photoresist masks. Good results are obtained with this technique in practice, if the diameters of the channels and the thickness of the dynode are approximately equal (see column 3, lines 5 to 10 of DE-PS No. 2,414,658).